1. Field of the Invention
The invention relates to a semiconductor substrate structure comprising a GaN-type layer stack of at least one buffer layer, a first active layer and a second active layer on top of a semiconductor substrate.
The invention also relates to a method of manufacturing such a semiconductor substrate structure and to devices made in such a semiconductor substrate structure.
2. Description of the Related Technology
GaN is a promising candidate for use for power and RF applications as well as for optical applications. Examples hereof include the manufacture of light emitting diodes, power amplifiers, power converters. Typical examples of GaN devices are a InGaN/GaN light emitting diode and an AlGaN/GaN HEMT device, i.e. a transistor including a source, a drain, a gate and a channel.
Typically, a silicon substrate is used as a carrier and growth template of the GaN layers. Good results have been achieved with substrates with a top layer of SiC in a (001) orientation and Si of a (111) orientation. Such Si (111) semiconductor substrate is advantageously provided as part of an SOI-type structure further comprising a handling wafer and a buried insulating layer. A GaN-type layer stack comprising a buffer layer, a first and a second active layer is grown thereon. Typically the GaN-type layer stack includes a nucleation layer. The layers in the stack typically contain nitrides of gallium (Ga), aluminum (Al), indium (In) and combinations thereof, e.g. AlGaN, InAlN, InGaN. The relative content of Ga and Al may vary across the layers. Other elements may be present as well or alternatively, such as known to a person skilled in the art. The layer stack may be grown as a continuous layer or as a discontinuous layer.
One of the problems of the use of a Si layer as a template for GaN-type layer stacks is the reliability. Particularly due to the difference in thermal expansion between silicon and gallium nitride, formation of cracks as well as relatively high defect densities have been found. Reliability tends to be an issue due to such cracks and defects particularly in the heavier use conditions for which the GaN-type stacks are most suitable. In order to reduce the mismatch problem, it is known to pattern the top layer of the substrate and/or the buffer layer in the GaN-type stack. The subsequent growth of the active layers then effectively occurs on localized islands.
U.S. Pat. No. 6,265,289 discloses such a method of localized growth of GaN, so as to reduce the defect density. The method starts with the provision of a layer stack of an AlN buffer and a GaN layer on top of a SiC substrate. Trenches are provided in this layer stack, the remaining portions of the layer stack being posts. These trenches may extend into the buffer layer or into the top layer of SiC. Subsequently, GaN is grown laterally from the posts into the trenches. This results in coalescence of the lateral growth fronts to obtain a continuous layer. At the bottom of the trench, a void may remain. Material simultaneously formed by vertical overgrowth of the posts is thereafter removed and replaced by GaN formed in a second lateral growth process. This method is however disadvantageous, for instance from a manufacturing perspective, due to the substantial growth time.
IEEE Electron Device Letters, 26 (2005), 130-132 discloses such a method of localized growth of GaN. This prior art method starts with the formation of a rectangular ridge in the Si(111) substrate, e.g. by removing the top layer around the ridge. A GaN-type layer stack is formed on the ridge. It is reported that the layer stack on the ridge is formed crack-free with a GaN buffer layer as thick as 1.5 μm. A polyimide based planarization technique is used for connecting the active device to pads of probing electrodes. In this technique a trench around the ridge is filled with polyimide. A conductor may be defined thereon. However, such polyimide technique is not a standard technology in semiconductor processing. Moreover, the polyimide may not be resistant to the rather stringent use conditions for GaN devices, in which particularly a lot of heat may be generated.
Although the methods of localized growth of GaN-type stacks reduce cracks and defect density, the resulting device appear insufficient for use in a power converter, such as a DC-DC converters, an AC-DC converters or other high voltage high efficiency switching devices. Particularly, the breakdown voltage appears insufficient. Typical breakdown requirements thereof include a breakdown voltage of 600V or 1000V in combination with a leakage of less than 1 μA/mm at 80% of the breakdown voltage. The breakdown voltage is therein specified as the voltage at which the leakage current is 1 mA/mm.
There is therefore a need for an improved substrate structure in which also power devices meeting such harsh requirements can be defined. Such substrate structure may as well be very suitable for other markets in which GaN is an interesting candidate.
It is therefore desirable to provide an alternative substrate structure and an alternative manufacturing method thereof suitable for higher breakdown voltages.
It is also desirable to provide an improved semiconductor device and an improved manufacturing method thereof.